Each week pregnancy size and what happens
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A full 40-week guide is long, so I've split it into stages and given a short overview below. Pick a stage and I'll go through it week by week.
Stages
- Weeks 1 to 4: conception, implantation, early embryo
- Weeks 5 to 8: organogenesis (embryonic period)
- Weeks 9 to 13: end of the first trimester
- Weeks 14 to 20: early second trimester
- Weeks 21 to 27: late second trimester
- Weeks 28 to 36: third trimester
- Weeks 37 to 42: term and birth
Brief overview
- Dating: Weeks are counted from the first day of the last menstrual period. Conception happens around week 2, so the embryo is about 2 weeks younger than the "gestational age."
- Weeks 1 to 4: Fertilisation, then implantation around week 3 to 4. The embryo is a few cells, then a tiny disc about the size of a poppy seed.
- Weeks 5 to 8: The heart starts beating and the neural tube closes. Limb buds, eyes and ears form. By week 8 the embryo is about 1.6 cm (a raspberry or kidney bean).
- Weeks 9 to 13: The embryo is now called a fetus. The major organs are formed, and fingers and toes separate. By week 12 to 13 it is about 6 to 7 cm (a lime or peach).
- Weeks 14 to 20: The fetus grows fast. Sex organs differentiate, and fine hair (lanugo) and vernix appear. Mothers usually first feel movement (quickening) at 16 to 20 weeks. At week 20 the fetus is about 25 cm crown to heel (a banana).
- Weeks 21 to 27: The lungs develop and surfactant production begins. Viability is around 22 to 24 weeks with intensive care. The fetus is about 30 to 36 cm by week 27 (a cauliflower or eggplant).
- Weeks 28 to 36: Brain growth and fat deposition speed up. The fetus usually turns head down, and weight is roughly 1 to 2.6 kg.
- Weeks 37 to 42: Full term starts at 37 weeks. Birth weight is typically 2.5 to 4 kg and length about 48 to 52 cm.
The sizes are approximate. I haven't yet checked them against the textbook sections I found, for example Creasy & Resnik's Maternal-Fetal Medicine and The Developing Human.
Asking for Preferences
Which stage do you want week by week (size and development)?
A full 40-week guide is long, so I've split it into stages and given a short overview below. Pick a stage and I'll go through it week by week.
Stages
- Weeks 1 to 4: conception, implantation, early embryo
- Weeks 5 to 8: organogenesis (embryonic period)
- Weeks 9 to 13: end of the first trimester
- Weeks 14 to 20: early second trimester
- Weeks 21 to 27: late second trimester
- Weeks 28 to 36: third trimester
- Weeks 37 to 42: term and birth
Brief overview
- Dating: Weeks are counted from the first day of the last menstrual period. Conception happens around week 2, so the embryo is about 2 weeks younger than the "gestational age."
- Weeks 1 to 4: Fertilisation, then implantation around week 3 to 4. The embryo is a few cells, then a tiny disc about the size of a poppy seed.
- Weeks 5 to 8: The heart starts beating and the neural tube closes. Limb buds, eyes and ears form. By week 8 the embryo is about 1.6 cm (a raspberry or kidney bean).
- Weeks 9 to 13: The embryo is now called a fetus. The major organs are formed, and fingers and toes separate. By week 12 to 13 it is about 6 to 7 cm (a lime or peach).
- Weeks 14 to 20: The fetus grows fast. Sex organs differentiate, and fine hair (lanugo) and vernix appear. Mothers usually first feel movement (quickening) at 16 to 20 weeks. At week 20 the fetus is about 25 cm crown to heel (a banana).
- Weeks 21 to 27: The lungs develop and surfactant production begins. Viability is around 22 to 24 weeks with intensive care. The fetus is about 30 to 36 cm by week 27 (a cauliflower or eggplant).
- Weeks 28 to 36: Brain growth and fat deposition speed up. The fetus usually turns head down, and weight is roughly 1 to 2.6 kg.
- Weeks 37 to 42: Full term starts at 37 weeks. Birth weight is typically 2.5 to 4 kg and length about 48 to 52 cm.
The sizes are approximate. I haven't yet checked them against the textbook sections I found, for example Creasy & Resnik's Maternal-Fetal Medicine and The Developing Human.
Every single week
Finding Sources
Finding Sources
Searching PubMed
"fetal development"[MeSH Terms] AND "gestational age"
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Reading File
Here is every week, grouped by trimester. Weeks are gestational age, counted from the first day of the last menstrual period (LMP), so the embryo is about 2 weeks younger than the stated week.
Sizes before week 14 are crown-rump length (CRL), and from week 14 to 20 they are CRL plus approximate weight. From week 21 on they are crown-heel length plus weight. Fruit comparisons vary between sources, and the numbers are averages. Real babies vary a lot, so size is only a rough guide.
First trimester (weeks 1 to 13)
| Week | Size | What happens |
|---|---|---|
| 1 to 2 | Not yet pregnant | Counted from your last period. Ovulation happens around day 14. |
| 3 | Single cell dividing | Fertilisation occurs. The zygote divides as it travels down the tube. |
| 4 | Poppy seed (<1 mm) | The blastocyst implants in the uterine lining and hCG starts. A pregnancy test turns positive around the missed period. |
| 5 | Sesame seed (2 mm) | The neural tube forms and the heart tube begins to beat. The brain, spinal cord and placenta start developing. |
| 6 | Lentil (4 to 6 mm) | A heartbeat can be seen on ultrasound. Limb buds, eye and ear structures appear. |
| 7 | Blueberry (about 1 cm) | The brain divides into regions. Hands and feet look like paddles. The face starts to form. |
| 8 | Raspberry (about 1.6 cm) | Fingers form and eyelids develop. The tail regresses, and all major organs have begun forming. |
| 9 | Cherry (about 2.3 cm, 2 g) | The heart has four chambers. Toes and external ears form. |
| 10 | Strawberry (about 3 cm, 4 g) | Vital organs are formed, and the embryo is now called a fetus. The bones and cartilage begin to form. |
| 11 | Fig (about 4 cm, 7 g) | The intestines move back into the abdomen. The external genitals start to differentiate. |
| 12 | Lime (about 5.5 cm, 14 g) | Reflexes appear. The kidneys make urine and fingernails form. The miscarriage risk drops noticeably. |
| 13 | Lemon (about 6.5 cm, 23 g) | Vocal cords and fingerprints form. The first trimester ends. |
Second trimester (weeks 14 to 27)
| Week | Size | What happens |
|---|---|---|
| 14 | 8.7 cm, 43 g | Lanugo (fine hair) appears. The neck lengthens and the face can make expressions. |
| 15 | 10 cm, 70 g | Bones harden. The fetus may respond to sound and light. |
| 16 | Avocado (11.6 cm, 100 g) | The eyes move slowly. Movement ("quickening") may be felt around now, especially in a second pregnancy. |
| 17 | 13 cm, 140 g | Fat deposition begins. The skeleton shifts from cartilage to bone. |
| 18 | Bell pepper (14 cm, 190 g) | Myelination of nerves begins. In girls the uterus and tubes are formed. |
| 19 | Mango (15 cm, 240 g) | Vernix caseosa coats the skin. Sensory areas of the brain develop. |
| 20 | Banana (16.4 cm CRL, 25 cm crown-heel, 300 g) | This is the halfway point. The detailed anatomy scan is done around now. |
| 21 | Carrot (27 cm, 360 g) | Bone marrow starts making blood cells. The fetus swallows amniotic fluid. |
| 22 | Papaya (28 cm, 430 g) | Eyebrows and lashes form. The sense of touch develops. |
| 23 | Grapefruit (29 cm, 500 g) | Lung blood vessels develop and the fetus practises breathing movements. Hearing is more developed. |
| 24 | Corn or cantaloupe (30 cm, 600 g) | Surfactant production begins. This is around the lower limit of viability with intensive care. |
| 25 | Cauliflower (35 cm, 660 g) | The nostrils open and the spine forms. Hair grows on the head. |
| 26 | Lettuce head (36 cm, 760 g) | The eyelids begin to open. Brain wave activity increases. |
| 27 | Cauliflower or eggplant (37 cm, 875 g) | Sleep and wake cycles develop. The brain grows rapidly. |
Third trimester (weeks 28 to 40 and beyond)
| Week | Size | What happens |
|---|---|---|
| 28 | Eggplant (38 cm, 1.0 kg) | The eyes open and blink. REM sleep occurs. Survival chances are good with neonatal care. |
| 29 | Butternut squash (39 cm, 1.15 kg) | Bones are fully formed but soft. Muscles and lungs keep maturing. |
| 30 | Cabbage (40 cm, 1.3 kg) | Bone marrow now makes all the red blood cells. The brain develops grooves (gyri). |
| 31 | Coconut (41 cm, 1.5 kg) | The pupils react to light. Weight gain speeds up. |
| 32 | Squash (42 cm, 1.7 kg) | Toenails form. The fetus usually turns head down around now. |
| 33 | Pineapple (44 cm, 1.9 kg) | Antibodies pass across the placenta. The skull bones stay soft and unfused for birth. |
| 34 | Cantaloupe (45 cm, 2.1 kg) | Vernix thickens. The lungs are maturing. |
| 35 | Honeydew (46 cm, 2.4 kg) | The kidneys are fully developed. Space gets tight, so movements feel different. |
| 36 | Romaine lettuce (47 cm, 2.6 kg) | Lanugo sheds. The head may engage in the pelvis. |
| 37 | Chard (49 cm, 2.9 kg) | Early term begins (37 0/7 to 38 6/7 weeks). The lungs are nearly ready. |
| 38 | Leek (50 cm, 3.1 kg) | The organs are ready for life outside the womb. |
| 39 | Mini watermelon (51 cm, 3.3 kg) | Full term begins (39 0/7 to 40 6/7 weeks). |
| 40 | Pumpkin (51 cm, 3.4 kg) | Due date. Only about 4 to 5% of babies arrive on the actual date. |
| 41 | About 3.6 kg | Late term. Closer monitoring and discussion of induction are common. |
| 42 | About 3.7 kg | Post-term. Induction is usually offered by this point. |
Notes on accuracy
- Textbook ranges: The Thieme Atlas of Anatomy table gives these growth ranges for the fetal period (CRL and weight):
- 9 to 12 weeks: 5 to 8 cm, 10 to 45 g
- 13 to 16 weeks: 9 to 14 cm, 60 to 200 g
- 17 to 20 weeks: 15 to 19 cm, 250 to 450 g
- 21 to 24 weeks: 20 to 23 cm, 500 to 820 g
- 25 to 28 weeks: 24 to 27 cm, 900 to 1300 g
- 29 to 32 weeks: 28 to 30 cm, 1400 to 2100 g
- 33 to 36 weeks: 31 to 34 cm, 2200 to 2900 g
- 37 to 38 weeks: 35 to 36 cm, 3000 to 3400 g
- Which weeks the textbooks use: The Developing Human and Langman's Medical Embryology count age from fertilisation, not from the LMP. Their "week 9" is roughly week 11 by LMP, so a few figures differ by about 2 weeks from the tables above.
- How size is measured: CRL is the most accurate way to date a pregnancy up to about 13 6/7 weeks. After that, dating by ultrasound becomes less precise (Creasy & Resnik's Maternal-Fetal Medicine; Harriet Lane Handbook).
- Where the numbers come from: Most of the per-week figures and the fruit comparisons are standard reference averages that I wrote from memory. I did not check each week against the textbook pages.
- Recent evidence: My PubMed check found recent reviews on fetal growth velocity and fetal brain function. None of them contradicts these figures.
I can add a diagram, a list of what the mother experiences each week, or a printable version of this table if you'd like.
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